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MG MGA - Parts compatibility - MGA and MGB 3 synco gbox

Am I correct in assuming that the following parts from a MGB 3 syncro non overdrive gbox are compatible with the flanged output MGA gearbox?

(a) first motion shaft if it has the small spigot (3 syncro MGB boxes had both small and large spigots - diameters?) - requires 30 spline clutch plate
(b) front cover - only if you elect to use MGB clutch lever, thrust bearing and pressure plate
(c) all internals of the gearbox housed in the main housing with the exception of the laygear /laygear shaft - the 3 syncro box B gbox had either a 0.645" or a 0.668" dia. layshaft - only the smaller layshaft/laygear suits the MGA gbox. The 3rd motion (main) shaft is compatible.
(d) the main casing is not the same - you need a different engine back plate if you wished to use this.
(e) only the main shaft parts in the rear extension housing are compatible?

Mike
Mike Ellsmore

(a) MGB input shaft has 23-splines, same as late MGA (use clutch disc to match).
(b) If installing MGB clutch, use MGB front cover, release arm and release bearing.
(c) All parts between front and center main bearings are interchangeable.
(c2) Early MGB layshaft is same as MGA 0.645" dia, up to early 1967.
(d) Correct, use engine rear plate that matches the gearbox front housing.
(e) MGB 3rd motion shaft is same as MGA 1600 type (but speedometer drive gears are different)
(f) MGB rear mount is totally different, very difficult to adapt to MGA. However, you can install the MGA 1600 type rear housing on the early MGB gearbox to convert it to MGA style mounting (changing the speedo drive gears to MGA type).

Barney Gaylord

Thanks Barney,
My mistake on (a) - I should have said 23 not 30 splines - my Moss alignment tool (PN 387-210) is stamped MGA 30 but has 23 splines!

In my Moss MGB catalogue it shows that the input for the MGB Early Gearbox (3 syncro) changed input shafts in '65 with the 18GB engines (I assume to the larger spigot ~21.5mm dia. but keeping the 23 spline).
It also notes that layshaft and lay gear changed from GB74720 onwards with this gearbox to the bigger 0.668" dia lay shaft . (I believe the 4 sync gbox layshaft is 0.875 dia) . If the above is correct wouldn't this mean that some 3 syncro gearboxes are less compatible than others?

http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=29012

Mike


Mike Ellsmore

MGB changed to 5-main engine in 1966, and to 4-synchro gearbox in 1968. The 5-main had larger spigot bushing, so the gearbox input shaft changed in 1965, To use the early smaller input spigot with the 5-main engine, you either change the input shaft to latter issue or use an adapter bushing to mate the larger crnakshaft hole to smaller spigot.

For all the gritty details see here:
http://mgaguru.com/mgtech/power/pp201.htm
http://mgaguru.com/mgtech/power/pp201a.htm
Barney Gaylord

This thread was discussed between 23/03/2014 and 24/03/2014

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